This invention relates in general to optical oscillators and in particular, to an optical oscillator whose frequency may be tuned so as to sweep across a predetermined bandwidth.
The use of optical fiber communications has expanded rapidly in recent years and has replaced microwave communications in many applications. For this reason much research and development have been performed in optical communications, particularly heterodyne communications systems employing optical heterodyne receivers. When an optical heterodyne receiver is tuned, a local oscillator is actually tuned instead of a filter. It is thus desirable to develop continuously swept optical oscillators which may be used in optical heterodyne receivers.
In microwave communication, microwave test and measurement instruments form an important part of the communications equipment. These test and measurement instruments include, for example, network analyzers and spectrum analyzers. The heart of both these instruments is a continuously swept oscillator. With the advent of optical communications, it is important to develop the optical analogs of the microwave network analyzers and spectrum analyzers. It is therefore desirable to provide a continuously swept optical oscillator. Other important microwave instruments containing tuneable oscillators are microwave synthesizers (accurate tuneable oscillators) and microwave sweepers (rapidly swept oscillators). Again it will be desirable to develop continuously swept optical oscillators which may be used as optical analogs of these instruments.
From the above, it is evident that it will be desirable to provide optical oscillator sweepers which may be used in the above-described optical instruments, including the heterodyne receiver, network analyzer, spectrum analyzer, synthesizers and sweepers.
For a brief review of various optical fiber communications schemes, see the article "Ultimate Performance of Heterodyne/Coherent Optical Fiber Communications," by T. Okoshi, Journal of Lightwave Technology, Vol. LT-4, No. 10, pp. 1556-1562 (Oct. 1986).
In the context of an optical heterodyne receiver, it is possible for the local optical oscillator to be tuned in discrete steps as long as the step size is precisely synchronized to the channel spacing. For example, discretely tunable external cavity lasers (ECL) utilizing semiconductor lasers and acoustooptic tunable filters (AOTFs) in linear feedback configurations have been reported by Coquin, G.A. and Cheung, K.W., in "Electronically tunable external cavity semiconductor laser," Electron. Lett., Vol. 24, pp. 599-600, 1988, and by Coquin et al. in "Single- and Multiple-Wavelength Operation of Acoustooptically Tuned Semiconductor Lasers at 1.3 .mu.m," IEEE Journal of Quantum Electronics, Vol. 25, No. 6, pp. 1575-1579 (June 1989).
Two problems arise, however, in designing such tunable optical oscillators. First, the combination of too much residual reflectivity from the anti-reflection (AR) coated laser facet and too wide an optical bandwidth for the AOTFs can prevent the optical oscillator from tuning at the external cavity frequency modes. The optical wavelengths that can be accessed may be restricted to the residual Fabry-Perot (F-P) modes of the semiconductor chip cavity, because when the AOTF is tuned in between the residual chip F-P modes, it has insufficient loss selectivity to overcome the gain maxima caused by constructive interference between the chip facets.
Second, even if the residual F-P modes of the semiconductor chip cavity did not prevent tuning to the external cavity modes, selective tuning between the discrete external cavity modes can be difficult. With this and other problems in mind, continuously tuned optical oscillators have been proposed. One such oscillator is proposed by Favre et al. in "External-Cavity Semiconductor Laser with 15nm Continuous Tuning Range," Electronics Letters, Vol. 22, No. 15, July 17, 1986, pp. 795-796. The laser proposed by Favre requires a grating to be continuously rotated and moved away or towards an amplifier chip in order to continuously tune the frequency of the laser. As explained in detail below, it is difficult to achieve the needed mechanical precision using mechanical tuning devices. It is therefore desirable to provide an improved continuously swept optical oscillator with improved characteristics.